1. Field of the Invention
The present invention relates to nonaqueous electrolyte secondary batteries and in particular to improvements in the cycle life, high-temperature storage stability and low-temperature discharge capacity characteristics of the batteries.
2. Description of Related Art
With recent rapid progress in the development of portable or cordless electronic equipment, there have been desired small and light-weight secondary batteries having a high energy density for use as electric sources for the driving of these electronic equipment. In this respect, nonaqueous electrolyte secondary batteries, especially lithium secondary batteries, are expected to have a high voltage and high energy density.
When the nonaqueous electrolyte secondary batteries are fabricated, the use of active materials for cathodes of high capacity and high voltage is required. As materials which meet these requirements, mention may be made of those which show a high voltage of 4 V, such as LiCoO.sub.2, LiNiO.sub.2, LiFeO.sub.2 and LiMn.sub.2 O.sub.4.
On the other hand, metallic lithium, lithium alloys and carbon materials capable of doping and undoping lithium ions are studied as anode materials. However, metallic lithium has a problem of short circuiting caused by dendrites produced by charging and discharging, and lithium alloys have a problem of collapse of electrodes due to expansion and contraction caused by charging and discharging. Accordingly, recently, carbon materials which have no such problems have been considered as anode materials of lithium secondary batteries.
It has been well known that when metallic lithium is used as anode materials, active dendrites produced on the surface of the anode in charging react with a nonaqueous solvent to bring about the decomposition of a part of the solvent, resulting in the reduction of charging efficiency. To solve this problem, Japanese Patent KOKAI (Laid-Open) No. Sho 57-170463 has proposed to use a mixed solvent comprising ethylene carbonate and propylene carbonate in view of the excellent charging efficiency of ethylene carbonate. Japanese Patent KOKAI (Laid-Open) No. Hei 3-55770 has proposed to use a mixture of a mixed solvent comprising ethylene carbonate and diethyl carbonate with 2-methyltetrahydrofuran, 1,2-dimethoxyethane, 4-methyl-1,3-dioxorane or the like as a solvent for the nonaqueous electrolyte in order to improve the low-temperature characteristics of batteries. Furthermore, U.S. Pat. No. 4,804,596 has proposed to use, as solvents for the nonaqueous electrolyte, those which contain esters such as methyl acetate and methyl formate as a base which are high in oxidation resistance and provide a high electric conductivity in order to improve high-rate discharge characteristics and low-temperature characteristics of the batteries.
However, even when these solvents are used, a charging efficiency obtainable is at most about 98-99% and a charging efficiency has not yet been sufficiently enhanced. This is the same when a lithium alloy is used for the anode.
When a carbon material is used for anode materials, the charging reaction is such that the lithium ions in the electrolyte are intercalated between layers of the carbon material so dendrites of lithium are not produced and thus the decomposition reaction of solvent on the surface of the anode as referred to above should not take place. However, actually, the solvent decomposes and charging efficiency does not reach 100%. Thus, there still remain the same problems as when lithium or a lithium alloy is used for anode.
Generally, a requirement for an excellent solvent of electrolyte for lithium batteries is that the solvent have a high dielectric constant, namely, it can dissolve a large amount of an inorganic salt which is a solute. Cyclic esters such as propylene carbonate, ethylene carbonate, .gamma.-butyrolactone and .gamma.-valerolactone are said to be excellent solvents which satisfy the above requirement, but still have the above problem that when carbon materials are used for anode materials, the decomposition reaction of solvent takes place in charging as mentioned above. Moreover, since the solvents above are highly viscous, there are problems that when they are used alone, the viscosity of the electrolytes is high and there is a difficulty in charging and discharging at a high rate and besides, a discharge capacity at low temperatures is low. Especially, ethylene carbonate has a high freezing point of 36.4.degree. C. and this cannot be used alone.
On the other hand, the decomposition reaction of non-cyclic carbonates in charging does not readily occur because of their structure, but these solvents have the defect that they are relatively low in dielectric constant and dissolve, with difficulty, the inorganic salt which is a solute and have the problem that many of them are relatively low in boiling point and so they are difficult to handle in the fabrication of batteries.
Furthermore, when these cyclic and non-cyclic carbonates are used in admixture, the above-mentioned problems in the case of using these compounds each alone are diminished, and excellent charge-discharge cycle life characteristics can be obtained but high-rate charge and discharge characteristics and low-temperature charge and discharge characteristics are insufficient. Usually, ethers of low freezing point and low viscosity are added to solvents used for electrolytes in order to improve the low-temperature characteristics of lithium batteries. However, ethers are generally low in oxidative decomposition voltage and when the above-mentioned lithium-containing oxides having a high voltage such as LiCoO.sub.2, LiNiO.sub.2, LiFeO.sub.2 and LiMn.sub.2 O.sub.4 are used as active materials for cathode, the decomposition reaction of the solvents is brought about in charging of batteries.